The objective of the studies to be undertaken in this proposal are: 1) to prepare radioimmunoassay kits using the Bolton-Hunter conjugation method for I125 labelling of erythropoietin (EP) for the purpose of undertaking a cooperative RIA to compare the assay of human serum and urinary erythropoietin in the RIA and polycythemic mice; 2) to undertake collaborative studies to isolate erythropoietin receptors in erythroid cells in rabbit fetal livers using purified I125 labelled Ep; 3) to determine the physiologic pathophysiologic roles of prostaglandins (PG) in kidney production of erythropoietin; 4) to determine whether adenylate cyclase is a kidney receptor for prostaglandins in the production of erythropoietin; and 5) to study the relationship of the chemical structures and the erythropoietic activities of several endoperoxides and prostanoids (prostaglandins analogs). Inhibitors of prostaglandins synthesis, which act upon cyclo-oxygenase (indomethacin and meclofenamate) and endoperoxide isomerase (p-mercuribenzoate), will be used to block PG production at different steps in the biosynthesis of renal PG's following erythropoietic stimulation with arachidonate (precursor of postaglandins), hypoxia and anemia. The receptor antagonist to PGE2, 7-oxa-13-prostynoic acid, which blocks PG at its renal tissue receptor site, will also be used to determine whether PGE2 is the active agonist at the receptor site. Studies are outlined using a renal cortical broken cell preparation of adenylate cyclase and theophylline (phosphodiesterase inhibitor) to attempt to relate the prostaglandins action to activation of adenylate cyclase in the kidney as a first step in initiating a cascade of events leading to the production of erythropoietin. The effects of arachidonate and some prostaglandins analogs on total circulating red cell mass in the rabbit will also be studied.